1. Field of the Invention
The present invention relates to a piston and piston-ring assembly. More particularly, the invention relates to a piston and piston-ring assembly where agglutination of aluminum alloy of the piston to a piston-ring is suppressed.
2. Description of Related Art
An assembly of a piston and a piston-ring for use in an internal combustion engine for a vehicle is required to have a heat resistive property. The piston usually comprises an aluminum base alloy for the purpose of making the piston light. When the engine is operated and the temperature of the piston and piston-ring assembly becomes high, the aluminum base alloy of the piston decreases in hardness and tends to agglutinate to the piston-ring, causing the piston-ring to stick to the piston-ring groove. Once the sticking happens, the ability of the piston to follow the cylinder bore surface is reduced, resulting in severe abrasion of the piston-ring and cylinder bore. Further, the severe abrasion will increase the temperature of the piston-ring, especially the temperature of the portion of piston-ring where the circumferential ends of the piston-ring butt together, and may cause a local melting of the piston-ring groove wall. Thus, sticking of the piston-ring to the piston should be prevented.
There are three typical prior art techniques for preventing such a piston-ring sticking. The first is to mold ceramic fibers in a portion of a piston close to a piston-ring groove, as is disclosed in Japanese Utility Model Publication SHO No. 59-81756. The second is to apply surface treatment such as an alumite treatment to a piston-ring groove. The second technique may include the alumite treatment and an ion plating for further forming a titanium nitride layer on the hardened layer obtained through the alumite treatment, as is disclosed in Japanese Utility Model Publication SHO No. 62-137361. The third is to form, on a piston-ring, a coating layer comprising tetrafluoroethylene, which has a good lubricative property and a bad adhesive property, in order to prevent a foreign material including the aluminum alloy of the piston from agglutinating to the piston-ring, as is disclosed in Japanese Utility Model Publication SHO No. 60-82552.
Though not for the purpose of preventing agglutination of the aluminum alloy of a piston to a piston-ring, Japanese Utility Model Publication SHO No. 59-184346 discloses the technique for constructing the oil-ring of polyimide resin for the purpose of improving the lubricative property of the piston-ring.
However, the above-described techniques have the following problems.
When the method of molding ceramic fibers in a piston is used, it is necessary to substitute pressure molding for gravity molding for the purpose of causing the aluminum alloy to soak into the ceramic fibers. This increases the molding cost.
When the method of applying alumite treatment to a surface of a piston-ring groove is used, the surface is roughened by the alumite treatment and the roughened surface will increase the amount of gas and oil passing through clearances formed in the roughened surface to increase the blowby gas amount and the oil consumption.
When the method of forming a coating layer of tetrafluoroethylene on a piston-ring is used, a severe abrasion will happen, once the coating layer is worn off the piston-ring. Thus, the method is less reliable in durability than the method of applying alumite treatment to a surface of a piston-ring groove.
Though the method of constructing an entire ring of polyimide resin can be applied to an oil ring which is exposed to a comparatively low temperature, the method can not be applied to a top piston-ring which is exposed to a high temperature.